High speed, high resolution, three dimensional solar cell inspection system

ABSTRACT

An optical inspection system and method are provided. A workpiece transport moves a workpiece in a nonstop manner. An illuminator includes a light pipe and is configured to provide a first and second strobed illumination field types. First and second arrays of cameras are arranged to provide stereoscopic imaging of the workpiece. The first array of cameras is configured to generate a first plurality of images of the workpiece with the first illumination field and a second plurality of images of the feature with the second illumination field. The second array of cameras is configured to generate a third plurality of images of the workpiece with the first illumination field and a fourth plurality of images of the feature with the second illumination field. A processing device stores at least some of the first, second, third, and fourth pluralities of images and provides the images to an other device.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is based on and claims the benefit of U.S.Provisional Application Ser. No. 61/244,616, filed Sep. 22, 2009 andU.S. Provisional Application Ser. No. 61/244,671, filed on Sep. 22,2009; the present application is a Continuation-In-Part application ofU.S. patent application Ser. No. 12/864,110 filed Jul. 22, 2010; and thepresent application is a Continuation-In-Part application of U.S. patentapplication Ser. No. 12/564,131, filed Sep. 22, 2009. All applicationslisted above are herein incorporated by reference in their entireties.

COPYRIGHT RESERVATION

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. The copyright owner has noobjection to the facsimile reproduction by anyone of the patent documentor the patent disclosure, as it appears in the Patent and TrademarkOffice patent file or records, but otherwise reserves all copyrightrights whatsoever.

BACKGROUND

Advancements in automated photovoltaic solar cell manufacturing areenabling higher throughput, yield, and cell conversion efficiencies. Forexample, commercially available automated equipment for applyingconductive layers to crystalline silicon solar cells routinely screenprint the metallization at a rate of one cell per second. Newertechnologies for improving cell conversion efficiencies such as theselective emitter process, metal-wrap-through, and print-on-print arebeing adopted that require precise registration of the metallizationlayers. Cell efficiencies are also affected by the height to width ratioof the metalized collector fingers, which collect the electric currentgenerated by the solar cell. These fingers must be printed narrowly inwidth to avoid unnecessary shading of the cell active area, but mustalso be printed tall in height to improve electrical conductivity. Also,the fragility of the thin silicon solar cells and their tendency to bowduring manufacturing, presents challenges to the automated handlingequipment to avoid chips and cracks. Bowed wafers may crack, forexample, when they are vacuum secured during one of the manymanufacturing process steps or when pressure is applied to the waferduring the screen printing process. In view of these industry demands, aneed has arisen for automated optical inspection systems that aredistributed throughout the solar cell manufacturing process to ensurehigh process yield. Given the increased needs for precisionregistration, narrower and taller features, and detection of waferbowing, it would be beneficial to provide an automated opticalinspection system that was not only faster than the prior art, but alsobetter able to provide higher resolution two and three dimensionalinspection of the solar cells.

SUMMARY

An optical inspection system and method are provided. A workpiecetransport is configured to transport a workpiece in a nonstop manner. Anilluminator is configured to provide a first strobed illumination fieldtype and a second strobed illumination field type. The illuminatorincludes a light pipe having a first end proximate the workpiece, and asecond end opposite the first end and spaced from the first end. Thelight pipe also has at least one reflective sidewall. The first end hasan exit aperture and the second end has at least one second end apertureto provide a view of the workpiece therethrough. A first array ofcameras is configured to digitally image the workpiece. The first arrayof cameras is configured to generate a first plurality of images of theworkpiece with the first illumination field and a second plurality ofimages of the feature with the second illumination field. A second arrayof cameras configured to digitally image workpiece. The second array ofcameras is configured to generate a third plurality of images of theworkpiece with the first illumination field and a fourth plurality ofimages of the feature with the second illumination field. The first andsecond arrays of cameras are configured to provide stereoscopic imagingof the workpiece. A processing device is operably coupled to theilluminator and the first and second arrays of cameras. The processingdevice is configured to store at least some of the first, second, third,and fourth pluralities of images and provide the first, second, thirdand fourth pluralities of images to an other device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional elevation view of an automated high speedoptical inspection system with a camera array and compact, integratedilluminator in accordance with embodiment of the present invention.

FIG. 2 is a diagrammatic elevation view of a plurality of cameras havingoverlapping fields of view in accordance with an embodiment of thepresent invention.

FIG. 3 is a system block diagram of an inspection system in accordancewith an embodiment of the present invention.

FIG. 4 is a top plan view of a transport conveyor, solar cell, and acamera array field of view acquired with a first illumination fieldtype.

FIG. 5 is a top plan view of a transport conveyor, solar cell, and acamera array field of view acquired with a second illumination fieldtype.

FIGS. 6A-6D illustrate a workpiece and camera array fields of viewacquired at different positions and under alternating first and secondillumination field types in accordance with an embodiment of the presentinvention.

FIG. 7 is a coordinate system for defining illumination direction.

FIG. 8 is a perspective view of a known linear line source illuminatinga camera array field of view.

FIG. 9 is a polar plot of the illumination directions of the illuminatorshown in FIG. 8.

FIG. 10 is a perspective view of an example hollow light pipeilluminator in accordance with an embodiment of the present invention.

FIG. 11 is a polar plot of the input illumination direction of theilluminator shown in FIG. 10.

FIG. 12 is a polar plot of the output illumination directions of theilluminator shown in FIG. 10.

FIG. 13 is a perspective view of a reflective surface of a light pipewall in accordance with an embodiment of the present invention.

FIGS. 14A-B are cross sectional views of the reflective surface shown inFIG. 13

FIG. 15A is a perspective view of a light pipe illuminator and cameraarray in accordance with an embodiment of the present invention.

FIG. 15B is a cutaway perspective view of a light pipe illuminator andcamera array in accordance with an embodiment of the present invention.

FIG. 16 is a cutaway perspective view of a camera array and illuminatorwith multiple sources in accordance with an embodiment of the presentinvention. FIG. 17A is a perspective cutaway view of an illuminator andcamera array in accordance with an embodiment of the present invention.

FIG. 17B is a cross sectional view of a chevron shaped mirror employedin accordance with an embodiment of the present invention.

FIG. 18 is a cutaway perspective view of an illuminator and camera arrayin accordance with an embodiment of the present invention.

FIG. 19 is a second cutaway perspective view of the illuminator andcamera array shown in FIG. 18.

FIG. 20 is a polar plot of the illumination directions of theilluminator shown in FIGS. 18 and 19.

FIG. 21 is a cross-sectional perspective view of an inspection sensor inaccordance with an embodiment of the present invention.

FIG. 22 is a polar plot of the illumination directions of theilluminator shown in FIG. 21.

FIG. 23 is a perspective view of two camera arrays arranged in a stereoconfiguration in accordance with an embodiment of the present invention.

FIG. 24 is a cutaway perspective view of two camera arrays arranged in astereo configuration with an integrated illuminator in accordance withan embodiment of the present invention.

FIG. 25 is a perspective view of two camera arrays and a structuredlight projector arranged in accordance with an embodiment of the presentinvention.

FIG. 26 is a perspective view of two camera arrays and a structuredlight projector arranged in accordance with an embodiment of the presentinvention.

FIG. 27 is a perspective view of a camera array and structured lightprojector in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Embodiments of the present invention generally provide a compactinspection system and method with high speed acquisition of multipleillumination two and three dimensional images without the need forexpensive and sophisticated motion control hardware. Processing of theimages acquired with different illumination types may appreciablyenhance the inspection capabilities and results.

FIG. 1 shows a cross-sectional elevation view of a system for generatinghigh contrast, high speed digital images of a workpiece that aresuitable for automated inspection, in accordance with an embodiment ofthe present invention. Camera array 4 consists of cameras 2A through 2Hpreferably arranged at regular intervals. Each camera 2A through 2Hsimultaneously images and digitizes a rectangular area on a workpiece orsubstrate, such as silicon photovoltaic solar cell 12, while theworkpiece undergoes relative movement with respect to cameras 2A through2H. Illuminator 45 provides a series of pulsed, short durationillumination fields referred to as strobed illumination. The shortduration of each illumination field effectively “freezes” the image ofsolar cell 12 to suppress motion blurring. Two or more sets of imagesfor each location on solar cell 12 are generated by camera array 4 withdifferent illumination field types for each exposure. Depending on theparticular features on solar cell 12 that need to be inspected, theinspection results may be appreciably enhanced by joint processing ofthe reflectance images generated with different illumination fieldtypes. Further details of illuminator 45 are provided in the discussionof FIGS. 21 and 22.

Workpiece transport conveyor 26 translates solar cell 12 in the Xdirection in a nonstop mode to provide high speed imaging of solar cell12 by camera array 4. Conveyor 26 includes belts 14 which are driven bymotor 18. Optional encoder 20 measures the position of the shaft ofmotor 18 hence the approximate distance traveled by solar cell 12 can becalculated. Other methods of measuring and encoding the distancetraveled of solar cell 12 include time-based, acoustic or vision-basedencoding methods. By using strobed illumination and not bringing solarcell 12 to a stop, the time-consuming transport steps of accelerating,decelerating, and settling prior to imaging by camera array 4 areeliminated. It is believed that the time required to entirely capturetwo complete 80 megapixel images of solar cell 12 of dimensions 156mm×156 mm, with two illumination field types, can be accomplished inapproximately one second or less

FIG. 2 shows the Y dimension location of each field of view 30A through30H on solar cell 12 that is imaged by cameras 2A through 2H,respectively. There is a slight overlap between adjacent fields of viewin order to completely image all locations on solar cell 12. During theinspection process, the images of discrete fields of view 30A through30H are digitally merged, or stitched, into one continuous image in theoverlap regions. Example camera array 4 is shown in FIGS. 1 and 2arranged as a single dimensional array of discrete cameras. As shown,cameras 2A-2H are configured to image in a non-telecentric manner. Thishas the advantage that the fields of view 30A through 30H can beoverlapped. However, the magnification, or effective resolution, of anon-telecentric imaging system will change as solar cell 12 thicknessvaries as well as the amount of bowing. Effects of solar cell 12warpage, thickness variations and other camera alignment errors can becompensated by image stitching. In another embodiment, the camera arraymay be arranged in a two dimensional array. For example, the discretecameras may be arranged into a camera array of two columns of fourcameras where adjacent fields of view overlap. Other arrangements of thecamera array may be advantageous depending on cost, speed, andperformance goals of the inspection system, including arrays where thefields of view do not overlap. For example, a staggered array of cameraswith telecentric imaging systems may be used.

FIG. 3 is a block diagram of inspection system 92. Inspectionapplication program 71 preferably executes on system computer 76. Inputsinto inspection program 71 include, for example, solar cell 12 geometry,metallization print geometry, and inspection tolerance limits for printdefects, color defects, edge chipping, microcrack size, and saw marks.Lighting and camera calibration data may also be input to inspectionprogram 71.

Inspection program 71 configures programmable logic controller 22 viaconveyor interface 72 with the transport direction and velocity of solarcell 12. Inspection program 71 also configures main electronics board 80via PCI express interface with the number of encoder 20 counts betweeneach subsequent image acquisition of camera array 4. Alternatively, atime-based image acquisition sequence may be executed based on the knownvelocity of solar cell 12. Inspection program 71 also programs orotherwise sets appropriate configuration parameters into cameras 2A-2Hprior to an inspection as well as strobe board 84 with the individualflash lamp output levels.

Panel sensor 24 senses the edge of solar cell 12 as it is loaded intoinspection system 92 and this signal is sent to main board 80 to beginan image acquisition sequence. Main board 80 generates the appropriatesignals to begin each image exposure by camera array 4 and commandsstrobe board 84 to energize the appropriate flash lamps 87 and 88 at theproper time. Strobe monitor 86 senses a portion of light emitted byflash lamps 87 and 88 and this data may be used by main electronicsboard 80 to compensate image data for slight flash lamp outputvariations. Image memory 82 is provided and preferably contains enoughcapacity to store all images generated for at least one solar cell 12.For example, in one embodiment, each camera in the array of cameras hasa resolution of about 5 megapixels and memory 82 has a capacity of about2.0 gigabytes. Image data from cameras 2A-2H may be transferred at highspeed into image memory buffer to allow each camera to be quicklyprepared for subsequent exposures. This allows solar cell 12 to betransported through inspection system 92 in a nonstop manner andgenerate images of each location on solar cell 12 with at least twodifferent illumination field types. The image data may begin to be readout of image memory 82 into PC memory over a high speed electricalinterface such as PCI Express (PCIe) as soon as the first images aretransferred to memory 82. Similarly, inspection program 71 may begin tocompute inspection results as soon as image data is available in PCmemory.

The image acquisition process will now be described in further detailwith respect to FIGS. 4-6.

FIG. 4 shows a top plan view of transport conveyor 26 and solar cell 12.Cameras 2A-2H image overlapping fields of view 30A-30H, respectively, togenerate effective field of view 32 of camera array 4. Field of view 32is acquired with a first strobed illumination field type. Solar cell 12is transported by conveyor 26 in a nonstop manner in the X direction.Solar cell 12 preferably travels at a velocity that varies less thanfive percent during the image acquisition process, although largervelocity variations and accelerations may be accommodated. In onepreferred embodiment, each field of view 30A-30H has approximately 5million pixels with a pixel resolution of 17 microns and an extent of 34mm in the X direction and 45 mm in the Y direction. Each field of view30A-30H overlaps neighboring fields of view by approximately 3 mm in theY direction so that center-to-center spacing for each camera 2A-2H is 42mm in the Y direction. In another embodiment, camera array 4 consists ofonly 4 cameras 2A-2D. In this embodiment, camera array field of view 32has a large aspect ratio in the Y direction compared to the X directionof approximately 5:1.

FIG. 5 shows solar cell 12 at a location displaced in the positive Xdirection from its location in FIG. 4. For example, solar cell 12 may beadvanced approximately 15 mm from its location in FIG. 4. Effectivefield of view 33 is composed of overlapping fields of view 30A-30D andis acquired with a second illumination field type. FIGS. 6A-6D show atime sequence of camera array fields of view 31, 33, 34, and 35 acquiredwith alternating first and second illumination field types. It isunderstood that solar cell 12 is traveling in the X direction in anonstop fashion. FIG. 6A shows solar cell 12 at one X location duringimage acquisition for the entire solar cell 12. Field of view 31 isacquired with a first strobed illumination field type as discussed withrespect to FIG. 4. FIG. 6B shows solar cell 12 displaced further in theX direction and field of view 33 acquired with a second strobedillumination field type as discussed with respect to FIG. 5. FIG. 6Cshows solar cell 12 displaced further in the X direction and field ofview 34 acquired with the first illumination field type and FIG. 6Dshows solar cell 12 displaced further in the X direction and field ofview 35 acquired with the second illumination field type.

There is a small overlap in the X dimension between field of views 31and 34 in order to have enough overlapping image information in order toregister and digitally merge, or stitch together, the images that wereacquired with the first illumination field type. There is also smalloverlap in the X dimension between field of views 33 and 35 in order tohave enough overlapping image information in order to register anddigitally merge the images that were acquired with the secondillumination field type. In the embodiment with fields of view 30A-30Hhaving extents of 33 mm in the X direction, it has been found that anapproximate 5 mm overlap in the X direction between field of viewsacquired with the same illumination field type is effective. Further, anapproximate 15 mm displacement in the X direction between fields of viewacquired with different illumination types is preferred.

Images of each feature on solar cell 12 may be acquired with more thantwo illumination field types by increasing the number of fields of viewcollected and ensuring sufficient image overlap in order to register anddigitally merge, or stitch together, images generated with likeillumination field types. Finally, the stitched images generated foreach illumination type may be registered with respect to each other. Ina preferred embodiment, workpiece transport conveyor 26 has lowerpositional accuracy than the inspection requirements in order to reducesystem cost. For example, encoder 20 may have a resolution of 100microns and conveyor 26 may have positional accuracy of 0.5 mm or more.Image stitching of fields of view in the X direction compensates forpositional errors of the solar cell 12.

It is desirable that each illumination field is spatially uniform andilluminates from consistent angles. It is also desirable for theillumination system to be compact and have high efficiency. Limitationsof two prior art illumination systems, linear light sources and ringlights, will be discussed with reference to FIGS. 7-9. Linear lightsources have high efficiency, but poor uniformity in the azimuth angleof the projected light. Ring light sources have good uniformity in theazimuth angle of the projected light, but are not compact and have poorefficiency when used with large aspect ratio camera arrays.

FIG. 7 defines a coordinate system for illumination. Direction Z isnormal to solar cell 12 and directions X and Y define horizontalpositions on solar cell 12 or other workpiece. Angle β defines theelevation angle of the illumination. Angle γ redundantly defines theillumination ray angle with respect to normal. Angle α is the azimuthangle of the ray. Illumination from nearly all azimuth and elevationangles is termed cloudy day illumination. Illumination predominantlyfrom low elevation angles, β, near horizontal is termed dark fieldillumination. Illumination predominantly from high elevation angles, β,near vertical is termed bright field illumination. A good, generalpurpose, illumination system will create a light field with uniformirradiance across the entire field of view (spatial uniformity) and willilluminate from consistent angles across the entire field of view (angleuniformity).

FIG. 8 shows known linear light sources 48 illuminating camera arrayfield of view 32. Linear light source 48 can use an array of LEDs 46 toefficiently concentrate light on a narrow rectangular field of view 32.A disadvantage of using linear light sources 48 is that although thetarget receives symmetrical illumination from the two directions facingthe sources, no light is received from the directions facing the longaxis of the FOV.

FIG. 9 is a two axis polar plot showing illumination directions for thetwo linear light sources 48. The polar plot shows that strongillumination is received by camera array field of view 32 from thedirection nearest to light sources 48 (at 0 and 180 degree azimuthangles) and that no illumination received from the 90 and 270 degreesazimuth angle. As the azimuth angle varies between 0 and 90 the sourceelevation angle drops and the source subtends a smaller angle so lesslight is received. Camera array field of view 32 receives light whichvaries in both intensity and elevation angle with azimuth angle. Thelinear light sources 48 efficiently illuminate field of view 32, butwith poor uniformity in azimuth angle. In contrast, known ring lightshave good uniformity in azimuth, but must be made large in order toprovide acceptable spatial uniformity for large aspect ratio camerafield of 32.

Although a ring light could be used to provide acceptable uniformity inazimuth, the ring light would need to be very large to provideacceptable spatial uniformity for camera field of view 32 ofapproximately 170 mm in the Y direction. For typical inspectionapplications, it is believed that the ring light would need to be over500 mm in diameter to provide sufficient spatial uniformity. Thisenormous ring fails to meet market needs in several respects: the largesize consumes valuable space on the assembly line, the large lightsource is expensive to build, the illumination angles are not consistentacross the working field, and it is very inefficient—the light outputwill be scattered over a significant fraction of the 500 mm circle whileonly a slim rectangle of the solar cell is actually imaged.

An optical device, referred to as a light pipe, can be used to produce avery uniform light field for illumination. For example, U.S. Pat. No.1,577,388 describes a light pipe used to back illuminate a film gate.Conventional light pipes, however, need to be physically long to provideuniform illumination.

A brief description of light pipe principles is provided with respect toFIGS. 10-12. Embodiments of the present invention are then describedwith respect to FIGS. 13-17 that significantly reduce the length of alight pipe required for uniform illumination. In one embodiment, theinterior walls of the light pipe are constructed with reflectivematerials that scatter light in only one direction. In anotherembodiment of the present invention, the light pipes are configured withinput and output ports that allow simple integration of a camera arrayto acquire images of a uniformly and efficiently illuminated workpiece.

FIG. 10 shows illuminator 65 which consists of light source 60 and lightpipe 64. Hollow box light pipe 64 which, when used as described, willgenerate a uniform dark field illumination pattern. Camera 2 viewsworkpiece 11 down the length of light pipe 64 through apertures 67 and69 at the ends of the light pipe. A light source 60, for example an arcin a parabolic reflector, is arranged such that it projects light intothe entrance aperture 67 of light pipe 64 with internally reflectingsurfaces such that light descends at the desired elevation angle.Alternatively a lensed LED or other source may be used as long as therange of source elevation angles matches the desired range of elevationangles at workpiece 11. The light source may be either strobed orcontinuous. The fan of rays from light source 60 proceeds across thepipe and downward until it strikes one of the side walls. The ray fan issplit and spread in azimuth at the corners of the pipe but the elevationangle is preserved. This expanded ray fan then spreads out, strikingmany different side wall sections where it is further spread andrandomized in azimuth angle and largely unchanged in elevation angle.After a number of reflections all azimuth angles are present at exitaperture 68 and workpiece 11. Therefore all points on the target areilluminated by light from all azimuth angles but only those elevationangles present in the original source. In addition, the illuminationfield at workpiece 11 is spatially uniform. Note that the lateral extentof light pipe 64 is only slightly larger than the field of view incontrast to the required size of a ring light for the condition ofspatially uniform illumination.

FIG. 11 shows the polar plot of the illumination direction at thesource, a nearly collimated bundle of rays from a small range ofelevation and azimuth angles.

FIG. 12 is a polar plot of the rays at workpiece 11 and the angularspread of the source is included for comparison. All azimuth angles arepresent at workpiece 11 and the elevation angles of the source arepreserved.

As the elevation angle of light exiting illuminator 65 is the same asthose present in the source 60, it is relatively easy to tune thoseangles to specific applications. If a lower elevation illumination angleis desired then the source may be aimed closer to the horizon. The lowerlimit to the illumination angle is set by the standoff of the light pipebottom edge as light cannot reach the target from angles below thebottom edge of the light pipe. The upper limit to the illuminationelevation angle is set by the length of light pipe 66 since severalreflections are required to randomize, or homogenize, the illuminationazimuth angle. As elevation angle is increased there will be fewerbounces for a given length light pipe 64 before reaching workpiece 11.

The polygonal light pipe homogenizer only forms new azimuth angles atits corners, therefore many reflections are needed to get a uniformoutput If all portions of the light pipe side walls could spread orrandomize the light pattern in the azimuth direction, then fewerreflections would be required and the length of the light pipe in the Zdirection could be reduced making the illuminator shorter and/or widerin the Y direction.

FIGS. 13 and 14 illustrate an embodiment of the present invention withlight pipe side walls which diffuse or scatter light in only one axis.In this embodiment it is preferred that the azimuth angles of the lightbundle be spread on each reflection while maintaining elevation angles.This is achieved by adding curved or faceted, reflective surface 70 tothe interior surface of light pipe side wall 66 as shown in FIG. 13.Cross sectional views of side wall 66 are shown in FIGS. 14A and 14B.FIG. 14A demonstrates how a collimated light ray bundle 62 is spreadperpendicular to the axis of the cylindrical curvature on reflectivesurface 70. In FIG. 14B, the angle of reflection for light ray bundle 62is maintained along the axis of the cylindrical curvature on reflectivesurface 70. Hence, the elevation angle of the source is maintained sincethe surface normal at every point of reflector 70 has no Z component.The curved, or faceted, surface of reflective surface 70 creates a rangeof new azimuth angles on every reflection over the entire surface of thelight pipe wall 66 and therefore the azimuth angle of the source israpidly randomized. Embodiment of the present invention can be practicedusing any combination of refractive, diffractive and reflective surfacesfor the interior surface of light pipe side wall 66.

In one aspect, reflective surface 70 is curved in segments of acylinder. This spreads incoming light evenly in one axis, approximatinga one-dimensional Lambertian surface, but does not spread light in theother axis. This shape is also easy to form in sheet metal. In anotheraspect, reflective surface 70 has a sine wave shape. However, since asine wave shape has more curvature at the peaks and valleys and lesscurvature on the sides, the angular spread of light bundle 62 isstronger at the peaks and valleys than on the sides.

FIGS. 15A and 15B show the curved, reflective surfaces applied to theinterior surfaces of light pipe illuminator 41 for camera array 4. Lightpipe illuminator includes side walls 66 and light source 87. Theone-dimensional diffusely reflecting surfaces 70 randomize azimuthangles more rapidly than a light pipe constructed of planar, reflectiveinterior surfaces. This allows a more compact light pipe to be usedwhich allows camera array 4 to be closer to the workpiece. FIG. 15Bshows how light rays are randomized in azimuth angle after a smallnumber of reflections.

Light pipe illuminator 42 can be shortened in the Z direction comparedto illuminator 41 if multiple light sources are used. Multiple sources,for example a row of collimated LEDs, reduce the total number ofreflections required to achieve a spatially uniform source and hencereduce the required light pipe length. Illuminator 42 is illustratedwith light sources 87A-87E which may also be strobed arc lamp sources.

In another aspect of the present invention shown in FIGS. 17A-17B,illuminator 43 includes mirrors 67 that reflect portions of the inputbeam from source 87 to the desired source elevation angle. Like themultiple source embodiment, this also results in a spatially uniformlight field in a shorter light pipe. Mirrors 67 are placed betweencameras to avoid blocking the view of the target and at differentheights so that each mirror intercepts a portion of the light comingfrom source 67. Mirrors are shaped to reflect light at the desiredelevation angle and toward light pipe side walls 66 where the curved,reflected surfaces 70 rapidly randomize the source azimuth direction. Across sectional view of mirror 67 is shown in FIG. 17B. Mirror 67 maybe, for example, a flat mirror that is formed into a series of chevrons.

In another embodiment of the present invention, FIGS. 18 and 19illustrate illuminator 44 integrated with camera array 4. Light isinjected by source 88 into light mixing chamber 57 defined by mirrors 54and 55, top aperture plate 58, and diffuser plate 52. The interiorsurfaces of 54, 55, and 58 are reflective, whereas diffuser plate 52 ispreferably constructed of a translucent, light diffusing material.Apertures 56 are provided on top plate 58 and apertures 50 are providedon diffuser plate 52 such that cameras 2 have an unobstructed view ofthe workpiece. In order to more clearly visualize diffuser plate 52 andapertures 50, mirror 55 has been removed in FIG. 19, compared with FIG.18.

Light projected by source 88 is reflected by mirrors 54 and 55 andaperture plate 58. As the light reflects in mixing chamber 57, diffuserplate also reflects a portion of this light and is injected back intomixing chamber 57. After multiple light reflections within mixingchamber 57, diffuser plate 52 is uniformly illuminated. The lighttransmitted through diffuser plate 52 is emitted into the lower sectionof illuminator 44 which is constructed of reflective surfaces 70, suchas those discussed with reference to FIGS. 13 and 14. Reflectivesurfaces 70 preserve the illumination elevation angle emitted bydiffuser plate 52. The result is a spatially uniform illumination fieldat workpiece 12. FIG. 20 is a polar plot showing the output illuminationdirections of illuminator 44. Illuminator 44 creates an output lightfield, as shown in FIG. 20, which is termed cloudy day sinceillumination is nearly equal from almost all elevation and azimuthangles. The range of output elevation angles, however, can be controlledby the diffusing properties of diffuser plate 52.

FIG. 21 shows another embodiment of optical inspection sensor 94.Optical inspection sensor 94 includes camera array 4 and integratedilluminator 45. Illuminator 45 facilitates independently controlledcloudy day and dark field illumination. A dark field illumination fieldis produced on solar cell 12 by energizing light source 87. A cloudy dayillumination field is projected onto solar cell 12 by energizing lightsource 88. FIG. 22 shows the polar plot and illumination directions forthe cloudy day and dark field illuminations. In one aspect, sources 87and 88 are strobed to suppress motion blurring effects due to thetransport of solar cell 12 in a non-stop manner.

It is understood by those skilled in the art that the image contrast ofvarious object features vary depending on several factors including thefeature geometry, color, reflectance properties, and the angularspectrum of illumination incident on each feature. Since each cameraarray field of view may contain a wide variety of features withdifferent illumination requirements, embodiments of the presentinvention address this challenge by imaging each feature and location onworkpiece 12 two or more times, with each of these images captured underdifferent illumination conditions and then stored into a digital memory.In general, the inspection performance may be improved by using objectfeature data from two or more images acquired with differentillumination field types.

It should be understood that embodiments of the present invention arenot limited to two lighting types such as dark field and cloudy dayillumination field nor are they limited to the specific illuminatorconfigurations. The light sources may project directly onto workpiece12. The light sources may also have different wavelengths, or colors,and be located at different angles with respect to workpiece 12. Thelight sources may be positioned at various azimuthal angles aroundworkpiece 12 to provide illumination from different quadrants. The lightsources may be a multitude of high power LEDs that emit light pulseswith enough energy to “freeze” the motion of workpiece 12 and suppressmotion blurring in the images. Numerous other lighting configurationsare within the scope of the invention including light sources thatgenerate bright field illumination fields or transmit through thesubstrate of workpiece 12 to backlight features to be inspected. Forexample, since silicon is semi-transparent at near infrared wavelengths,it is especially effective to backlight solar cell 12 with strobed, nearinfrared light to inspect for microcracks and holes in the substrate.

Several solar cell inspection requirements necessitate the need tocapture three dimensional image data at full production rates. Theserequirements include measuring metallization print height and waferbowing. Three dimensional information such as the profile of a collectorfinger may be measured using well known laser triangulation, phaseprofilometry, or moiré methods, for example. U.S. Pat. No. 6,577,405(Kranz, et al) assigned to the assignee of the present inventiondescribes a representative three dimensional imaging system. Stereovision based systems are also capable of generating high speed threedimensional image data.

Stereo vision systems are well known. Commercial stereo systems date tothe stereoscopes of the 19^(th) century. More recently a great deal ofwork has been done on the use of computers to evaluate two camera stereoimage pairs (“A Taxonomy and Evaluation of Dense Two-Frame StereoCorrespondence Algorithms” by Scharstein and Szeliski) or multiplecameras (“A Space-Sweep Approach to True Multi-Image Matching” by RobertT. Collins). This last reference includes mention of a single cameramoved relative to the target for aerial reconnaissance.

An alternative stereo vision system projects a structured light patternonto the target, or workpiece, in order to create unambiguous texture inthe reflected light pattern (“A Multibaseline Stereo System with ActiveIllumination and Real-time Image Acquisition” by Sing Bing Kang, Jon A.Webb, C. Lawrence Zitnick, and Takeo Kanade).

To acquire high speed two and three dimensional image data to meet solarcell inspection requirements, multiple camera arrays may be arranged ina stereo configuration with overlapping camera array fields of view. Thesolar cell can then be moved in a nonstop fashion with respect to thecamera arrays. Multiple, strobed illumination fields effectively“freeze” the image of the solar cell to suppress motion blurring.

FIG. 23 shows camera arrays 6 and 7 arranged in a stereo configuration.Camera arrays 6 and 7 image solar cell 12 with overlapping camera arrayfields of view 37. The illumination system is removed for clarity.

FIG. 24 is a cutaway perspective view of optical inspection sensor 98with integrated illuminator 40 for high speed acquisition of stereoimage data. Camera arrays 3 and 5 are arranged in a stereo configurationwith overlapping fields of view 36 on solar cell 12. Solar cell 12 movesin a nonstop fashion relative to inspection sensor 98. Top apertureplate 59 includes apertures 56 and translucent diffuser plate 53includes apertures 50 to allow unobstructed views of field of view 36for camera arrays 3 and 5. Energizing light source 88 will create acloudy day illumination field type on solar cell 12 and energizing lightsource 87 will create a darkfield illumination field type. Otherillumination field types, such as backlight, may be accomplished bysuitable arrangement of a strobed illuminator such that lighttransmitted through, or past the edges, of solar cell 12 are captured bycamera arrays 3 and 5. The image acquisition sequence may be, forexample, a series of overlapped images captured simultaneously by bothcamera arrays 3 and 5 with alternating strobed cloudy day, darkfield,and backlight illumination field types.

Referring back to block diagram FIG. 3, the functional block diagram ofoptical inspection sensor 98 is very similar to the block diagram ofoptical inspection sensor 94. For optical inspection sensor 98, however,camera array 4 is removed and replaced by camera arrays 3 and 5 whichare in turn interfaced to main electronics board 80. Image memory 82preferably contains enough capacity to store all images generated bycamera arrays 3 and 5 for one solar cell 12. Image data is read out ofimage memory 82 and transferred to system computer 76 over a high speedelectrical interface such as PCI Express (PCIe).

Application inspection program 71 computes three dimensional image databy known stereo methods using the disparity or offset of image featuresbetween the image data from camera arrays 3 and 5. Inspection resultsare computed by application program 71 for solar wafer 12 properties anddefects such as wafer geometry, chipped edges, holes, cracks,microcracks, surface inspection, bow, saw marks, and color. Printinspection results for position, thickness, width, length, andinterruptions may also be computed by application program 71. Theregistration of the metalized print may also be enhanced by measuringfiducials, such as those that are laser etched onto the surface of solarcell 12. These fiducials often show good contrast in darkfieldilluminated images and may be used to establish a coordinate system formeasuring registration. A combination of two and/or three dimensionalimage data may be used for any of these inspection computations.

FIG. 25 shows another embodiment where camera arrays 6 an 7 are arrangedin a stereo configuration with overlapping camera array fields of view37 on solar cell 12. The integrated cloudy day and darkfield illuminatorhas been removed for clarity. Stereo vision systems sometimes fail inthe absence of observable structure on the object. A method ofovercoming this is to add artificial structure or “texture” to thesurface with a patterned light source that can then be viewed by camerasarranged in a stereo configuration. Structured light projector 8projects a strobed light pattern onto solar cell 12 over the cameraarray field of view 37. The light pattern may be, for example, a laserstripe, a series of laser stripes, or a random dot pattern. Thedisparity of the projected pattern as viewed by camera arrays 6 and 7may be used by application program 71 to compute three dimensional imagedata. The image acquisition sequence may be a series of overlappedimages captured simultaneously by both camera arrays 6 and 7 withalternating strobed cloudy day, darkfield, and structured light patternillumination field types.

FIG. 26 shows another embodiment with camera arrays 6 and 7 arranged ina stereo configuration and with structured light projector 8. Theintegrated cloudy day and darkfield illuminator has been removed forclarity. Camera array 6 is arranged to view solar cell 12 from avertical direction to eliminate the perspective view, as in FIG. 25, toimprove two dimensional measurement of solar cell 12 features.

FIG. 27 shows another embodiment with camera array 6 arranged to viewcamera array field of view 38 on solar cell 12. Structured lightprojector 8 projects a strobed light pattern onto solar cell 12 over thecamera array field of view 38. The light pattern may be, for example, alaser stripe, a series of laser stripes, a sinusoidal pattern, or arandom dot pattern. Range to solar cell 12 and its features arecalculated by known methods by measuring the position of the projectedlight pattern as observed by camera array 6. Optional cloudy day,darkfield, brightfield, backlight, or other light sources have not beenshown for clarity.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

1. An optical inspection system comprising: a workpiece transportconfigured to transport the workpiece in a nonstop manner; and anilluminator configured to provide a first strobed illumination fieldtype and a second strobed illumination field type, the illuminatorincluding a light pipe having a first end proximate the workpiece, and asecond end opposite the first end and spaced from the first end, thelight pipe also having at least one reflective sidewall, and wherein thefirst end has an exit aperture and the second end has at least onesecond end aperture to provide a view of the workpiece therethrough; afirst array of cameras configured to digitally image the workpiece,wherein the first array of cameras is configured to generate a firstplurality of images of the workpiece with the first illumination fieldand a second plurality of images of the workpiece with the secondillumination field; a second array of cameras configured to digitallyimage the workpiece, wherein the second array of cameras is configuredto generate a third plurality of images of the workpiece with the firstillumination field and a fourth plurality of images of the workpiecewith the second illumination field; wherein the first and second arraysof cameras are configured to provide stereoscopic imaging of theworkpiece; and a processing device operably coupled to the illuminatorand the first and second arrays of cameras, the processing device beingconfigured to provide at least some of the first, second, third, andfourth pluralities of images to an other device.
 2. The opticalinspection system of claim 1, wherein the first array of camerasincludes non-telecentric optics and wherein the cameras of the firstarray of cameras are aligned with one another along an axisperpendicular to a direction of workpiece motion and wherein the camerasof the first array of cameras have fields of view that overlap oneanother.
 3. The optical inspection system of claim 1, wherein the firstarray of cameras includes non-telecentric optics and wherein the camerasof the second array of cameras are aligned with one another along anaxis perpendicular to the direction of workpiece motion, but spaced fromthe first plurality of cameras in a direction of the workpiece motion.4. The optical inspection system of claim 1, wherein: the first array ofcameras has telecentric optics, and wherein the cameras of the firstarray of cameras are aligned with one another along an axisperpendicular to a direction of workpiece motion, and have fields ofview that do not overlap with one another; and further comprising athird array of cameras having telecentric optics, and wherein thecameras of the third array of cameras are aligned with one another alongan axis perpendicular to a direction of workpiece motion, and havefields of view that do not overlap with one another, wherein the firstand third arrays of cameras have fields of view that are staggered;wherein the second array of cameras has telecentric optics, and whereinthe cameras of the second array of cameras are aligned with one anotheralong an axis perpendicular to a direction of workpiece motion, and havefields of view that do not overlap with one another; and furthercomprising a fourth array of cameras having telecentric optics, andwherein the cameras of the fourth array of cameras are aligned with oneanother along an axis perpendicular to a direction of workpiece motion,and have fields of view that do not overlap with one another, whereinthe second and fourth arrays of cameras have fields of view that arestaggered.
 5. The optical inspection system of claim 1, and furthercomprising an encoder operably coupled to the workpiece transport toprovide an indication of workpiece motion to the processing device. 6.The optical inspection system of claim 5, wherein the indication has aresolution of approximately 100 microns.
 7. The optical inspectionsystem of claim 1, wherein the illuminator includes at least one arclamp.
 8. The optical inspection system of claim 1, wherein theilluminator includes at least one light emitting diode.
 9. The opticalinspection system of claim 1, wherein the light pipe includes aplurality of reflective sidewalls.
 10. The optical inspection system ofclaim 1, wherein the at least one reflective sidewall includes a curvedreflective surface that preserves illumination elevation angle whilemixing illumination azimuthally.
 11. The optical inspection system ofclaim 1, wherein the illuminator includes at least one mirror disposedto reflect at least a portion of illumination to a desired sourceelevation angle.
 12. The optical inspection system of claim 11, whereinthe at least one mirror is angled to reflect the portion of illuminationtoward the at least one reflective sidewall at the desired elevationangle.
 13. The optical inspection system of claim 1, wherein theilluminator includes an illumination mixing chamber disposed proximatethe second end, and wherein the mixing chamber and the light pipe areseparated by a translucent diffuser having at least one diffuseraperture aligned with each respective at least one second end aperture.14. The optical inspection system of claim 13, wherein a first lightsource is configured to introduce strobed illumination into the mixingchamber.
 15. The optical inspection system of claim 14, wherein a secondlight source is configured to introduce strobed illumination into thelight pipe between the diffuser and the first end.
 16. The opticalinspection system of claim 15, and further comprising a third lightsource configured to introduce additional illumination into the lightpipe between the diffuser and the first end.
 17. The optical inspectionsystem of claim 1, wherein the processing device includes a high-speeddata transfer bus to provide the first, second, third and fourthpluralities of images to the other device.
 18. The optical inspectionsystem of claim 17, wherein the processing device is configured tosimultaneously acquire and store images from the first and second arraysof cameras while providing images to the other device.
 19. The opticalinspection device of claim 18, wherein the high-speed data transfer busoperates in accordance with the peripheral component interconnectexpress (PCIe) bus.
 20. The optical inspection system of claim 1,wherein the other device is configured to provide an inspection resultrelative to the feature on the workpiece based, at least in part, uponthe first, second, third and fourth pluralities of images.
 21. Theoptical inspection system of claim 1, and further comprising astructured light projector configured to project structured light ontothe workpiece.
 22. The optical inspection system of claim 21, whereinthe structured light projector is configured to project a random dotpattern.
 23. The optical inspection system of claim 21, wherein thestructured light projector is configured to project at least one laserstripe.
 24. The optical inspection system of claim 21, wherein thestructured light projector is configured to project at least onesinusoidal fringe onto the workpiece.
 25. The optical inspection systemof claim 1, wherein the first and second arrays of cameras are angledwith respect to a surface normal to the workpiece.
 26. The opticalinspection system of claim 1, wherein the first array of cameras isoriented to view the workpiece from an angle that is substantiallyperpendicular to a surface of the workpiece.
 27. A method of inspectingan article of manufacture, the method comprising: generating relativemotion between the article of manufacture and a pair of camera arrays;simultaneously acquiring a first set of images from a first camera arrayof the pair of camera arrays through a light pipe during the relativemotion and while strobing a first illumination field type upon thearticle of manufacture; generating at least a first stitched image withthe first set of acquired images; acquiring a second set of images fromthe second camera array through the light pipe while strobing the firstillumination field type upon the article of manufacture; generating atleast a second stitched image from the second set of images acquiredfrom the second camera array; and determining the inspection resultrelative to the article of manufacture based, at least in part, upon thefirst and second stitched images.
 28. The method of claim 27, andfurther comprising: simultaneously acquiring a third set of images fromthe first camera array of the pair of camera arrays through a light pipeduring the relative motion and while strobing a second illuminationfield type upon the article of manufacture; generating at least a thirdstitched image with the third set of acquired images; acquiring a fourthset of images from the second camera array through the light pipe whilestrobing the second illumination field type upon the article ofmanufacture; generating at least a fourth stitched image from the fourthset of images acquired from the second camera array; and determining theinspection result relative to the article of manufacture based, at leastin part, upon the first, second, third and fourth stitched images. 29.The method of claim 28, wherein the first illumination field type isdarkfield.
 30. The method of claim 29, wherein the second illuminationfield type is cloudy day.
 31. The method of claim 29, wherein the secondillumination field type is brightfield.
 32. The method of claim 28,wherein the first illumination field type is structured illumination.33. The method of claim 28, wherein the first illumination field type isbacklight.
 34. The method of claim 33, wherein the backlight has anear-infrared wavelength.
 35. The method of claim 28, wherein the firstand second illumination field types are energized alternately.
 36. Themethod of claim 27, wherein the image stitching is used to correct forpositional error of the article of manufacture.
 37. The method of claim27, wherein stitching is used to correct for workpiece warpage.
 38. Themethod of claim 27, and further comprising providing at least someimages to an other device while collecting images from the pair ofcamera arrays.
 39. The method of claim 27, wherein the pair of cameraarrays are arranged to view the article stereoscopically, and whereinthe method further comprises calculating three dimensional surfaceinformation based upon at least one of the first and second sets ofimages.
 40. The method of claim 39, wherein the inspection result isbased on two dimensional and three dimensional image data.
 41. Themethod of claim 27, wherein the article of manufacture is a solar cell.42. The method of claim 41, wherein the article of manufacture is asolar cell and wherein the method further comprises establishing acoordinate frame using solar cell fiducial marks.
 43. The method ofclaim 41, wherein the inspection result is indicative of at least one ofcollector finger height, collector finger width, and collector fingerregistration.
 44. The method of claim 41, wherein the inspection resultis indicative of solar cell bowing.
 45. The method of claim 41, whereinthe inspection result is indicative of solar cell wafer geometry. 46.The method of claim 41, wherein the inspection result is indicative ofthe presence of chips in the solar cell.
 47. The method of claim 41,wherein the inspection result is indicative of the presence of cracks inthe solar cell.